The present invention generally relates to the field of power electronics, and more particularly to a novel pulse width modulation (PWM) technique and circuit for balancing the voltage of DC links of a cascaded-multilevel converter (CMC) for reactive power compensations. The invention has application in, for example, electric power generation in Flexible Alternating Current Transmission systems (FACTS) and motor drives, and is useful in both active and reactive power generation.
The method and circuit described and claimed in this application can be implemented by hardware circuitry. Thus, the calculation time in the digital signal processor (DSP) is only slightly increased as more voltage levels are employed. The basic structure of the proposed technique is modular. Therefore, the method and circuit may be employed for any number of H-bridge converters. With these features, the complexity of the DSP programming for the control loop is not affected by increasing the number of voltage levels.
Prior art DC capacitor voltage-balancing techniques add individual DC voltage loops into the main control loop. The compensators of each individual loop, however, are very difficult to design because of the complexity of the voltage-loop transfer functions. Thus, trial and error evaluation and testing provides the simplest way to achieve a satisfactory compensator design. This process is very time-consuming. Moreover, the greater the number of voltage levels, the more complex the control design. The main controller, which is the DSP-based, must perform all of the feedback control steps. As a result, the prior art approach potentially reduces the reliability of the controller.
To overcome the disadvantages of prior art devices and methods, this application describes a new PWM technique, which is specially designed for CMC-based Static Synchronous Shunt Compensators (STATCOM). The PWM method is referred to as a “cascaded PWM”, in which the DC capacitor voltages, the output currents and the three-phase multilevel duty cycle command are used as its inputs.